The stag beetle Lucanus cervus (LINNAEUS, 1758)

Text © Klaas Reißmann, 2007

1.   Introduction

With its bodylength of up to 80 mm, the distinctive stag beetle is the largest species of the German beetle fauna. Most people know the beetle, a few may even have found one themselves. Due to the large, eye-catching mandibles of the male, the beetle's vernacular name has been derived from the stag, e.g. as "stag beetle" in English, or "Hirschkäfer" in German. Nowadays not too many people have ever seen the stag beetle in the wild. In the Red List of the German beetle fauna (1998) the beetle is classified as "2" (endangered), due to the fact, that larger populations today are rather rare and limited to small regions. Currently there is no evidence for a positive development of the German stag beetle populations.

Lucanus cervus, male Until the thirties of the last century, the stag beetle was, at least locally, a common species. Although first losses in the populations were registered at that time already, our grandparents usually knew the beetle not only from stories, but from their own experience. So how could it happen, that the formerly common species in our forests is now more or less facing extinction?

To find an answer to this question, it is necessary to understand the life cycle of the stag beetle and the requirements for a successful reproduction and development of the larvae. At exactly this point, the experts faced a great challenge, because the reasons for the losses in the stag beetle populations were not explicable. It was known, that the larvae develop in decaying wood, mostly oak (Quercus), but occasionally beech (Fagus), apple (Malus), willow (Salix), poplar (Populus), chestnut (Aesculus), hawthorn (Crataegus) and several others are accepted as well. In managed forest, large trees are cut to sell the wood. Forestry offices often do the rest by removing dead trees for road safety reasons. In a typical stereotype, German forests have to be tidy, and for many years, dead trees have been removed for that reason. The culprit seems to be found: the forestry and the forestry offices.

But it's not that simple: The larvae of the stag beetle mainly develop in decaying wood of rootstocks. The above-ground part of the rootstock is only rarely used. And that was the most mysterious part of the phenomenon: Usually the rootstocks are the only remains of cut trees in the forest. Rootstocks were always abundant in German forests, and still are to this day. So our forests seem to be a loaded table.

Due to this apparent contradiction, E. TOCHTERMANN investigated the life cycle and conditions more closely, with astounding results. The hitherto known facts and TOCHTERMANN'S results shall be outlined on this page.

2.   Life cycle

Lucanus cervus, male In Central Europe, the imagines (adults) of the stag beetle appear in early summer, usually in June and July. Depending on the weather conditions, the first imagines may also appear already in May, especially in southern Germany, whereas "latecomers" and the more long-living females may be found until August. At higher elevation, the activity period is delayed accordingly, the imagines are found from July to August, sometimes even until September. The males usually hatch about one week earlier than the females, because their sealed cell is less deep located in the ground and grows warm earlier in spring.

This fact is indeed relevant for the survival of the species, because the males immediately start to frequent oaks, from which sap is oozing. They try to deter woodpeckers and other predators with their large mandibles. About one week later, the females hatch and start to frequent the oozing trees as well. TOCHTERMANN discovered, that pheromons are only of minor importance in the mate localization of stag beetles. Pheromons do only have an attracting effect at distances below 2 meters. However, the males are strongly attracted by the odor of sap oozing from oak trees.

click to view next picture In the proximity of such spots courtship behavior takes place, and - in case two or more males encounter each other - the males fight each other. In the fight, the males try to lever out the opponent with their mandibles and to throw each other off the tree. In such cases, the clicking noise of the elytrae can be heard from a few meters distance. Occasionally the opponents can even cause deadly injuries in the abdomen of their rival. The mating usually takes place directly on the spot, where sap is oozing from the bark, with the male adopting its typical protecting position over the female.

Upon mating, the male uses its large mandibles to retain the female. The couple may maintain this position for up to three days. During this time, the beetles mate repeatedly and feed on the fermeting sap oozing from the bark. The male adopts its typical mating position, covering the female against predators. Although the males can deter predators with their mandibles, usually they fall prey to woodpeckers and other birds first, whereas the females let themselves drop off the tree and often survive an attack. In this way the gender ratio between females and males drops from initally 1:3-4 females:males) within only three weeks to 1:1.5, at the very end of the activity period even to 1:0.5-0.7. Thus the death rate of the males through predation is much higher than of the females. Also fermenting tree sap claims many victims among the males. Due to the alcohol contained in the sap, the capacity of reaction in the males is reduced and consequently they fall prey more often.

Leftovers of stag beetles preyed by birds Typical predators of stag beetles are various bird species, like woodpeckers (Picidae), jaybirds (Garrulus glandarius) and owls (Strigidae), but also kestrels (Falco tinnunculus) and crows (Corvus) and other species. Crows and jaybirds have been observed to separate head, pronotum and elytrae before swallowing the abdomen. The remains can be often found during the activity period of the beetles. Wild hogs (Sus scrofa) and badgers (Meles meles) prey the pupae in the ground. Rumpled ground around rootstocks of oaks is a clear sign of wild hog activity in the area. Other predators like hedgehog (Erinaceus europaeus), fox (Vulpes vulpes), shrews and moles (Talpa europaea) feed on the beetles as well. Hence the mortality of the beetles is high. TOCHTERMANN found out, that only 100 females per square kilometer out of 150 achieve oviposition with 12 to 14 eggs, in total 1,200 to 1,400 eggs. Out of these only 600 larvae reach the imaginal stage. With a gender ratio of 1:4 (females:males) this means, that the number of females esentially remains constant, of course subject to seasonal fluctuation. After mating the females start their strenous search for suitable breeding substrates. Often they seek the same rootstock where they developed. Breeding substrate more than 2,000 m away from the mating spot is not frequented.

Lucanus cervus, female When the female finds suitable breeding substrate, it digs itself approximately 30 to 50 cm, at most 100 cm into the soil. It deposits 12 to 14 eggs close to the roots of the rootstock. Afterwards, the female frequents again mating spots. The imgaginal life of the female therefore can be regarded as a cycle of feeding, mating, searching for breeding substrate and oviposition. About 70% of their lifetime is spent for the search of suitable breeding substrate. About 50% of the females achieve a 2nd oviposition, only 5% a 3rd one. In captivity also a 4th oviposition has been observed. The maximal number of eggs produced by one female has been 28. Formerly it was believed, that a female can produce 50 to 100 eggs, which seems doubtful for sheer anatomic reasons. However, breeding observations with species from other families, like some jewel beetles (Buprestidae), indicate, that the food intake of the female provides the basis for a sustained production of eggs (ZÁBRANSKÝ, P., personal correspondence, 2007). Often oviposition is not achieved at all, for example if the female cannot find suitable breeding substrate. In this case they dig themselves into the ground, come out without having achieved anything, continue their search, dig themselves in and so on, until they die from exhaustion, and the eggs in their abdomen perish.

Directly after oviposition the egg is approx. 2 mm in diameter. Within 20 days it swells to the size of a pea. The larva feeds in wet, decaying wood. A L3 larva consumes approx. 250 cm3 substrate per month (TOCHTERMANN, 1997). In the literature different numbers for the duration of the larval stage have been given, but in Germany 5 to 6 years are common, seldom it takes even longer (up to 8 years). During this time the larva has reached 10 cm of body lenght. At the end of the larval stage, the larva leaves the wood and digs itself into the soil. The beetle hatches already in October, but remains in its pupal cell as the chitinous exosceleton needs to harden. Hibernation takes place in the pupal cell as well.

In the following summer, the new beetles emerge from their pupal cell to the surface and the development cycle is complete.

3.   Population losses and causes

Up to this point the cause for the massive losses in the populations remains unclear. However, recent research brought up an unexpected explanation: There is a connection between the requirements of the larvae regarding its substrate, and the cellular components of oak wood.

For a successful development, the larva of the stag beetle requires suitable wood, mostly oak wood. It is essential, that the celluar components, especially lignin, is enzymatically cleaved into simpler chemical compounds. This usually happens through the enzymatic activity of fungi, which cause white or red oak rot, e.g. the fungus species Deadalea quercina, Fomitopsis pinicola, Phellinus robustus and Fistulina hepatica. Up to 5 cm depth below the wood surface these fungi create enough substrate suitable for tha larva. The fungi are found in every forest, so in priciple the prerequisites for the existence of the stag beetle should be fulfilled.

Lampertheim forest (South Hesse) in winter Oak wood contains up to 18 different tannic acids. During the growth period the tannic acids are equally distributed among to whole tree. However, in autumn the tannic acids are concentrated in the roots of the oak. As trees are usually cut in late autumn or winter in Germany, the rootstock remaining after cutting contains high amounts of tannic acid, hindering fungal activity. Although the fungi are still present on the wood, the wood is only digested up to 0.5 cm below the surface. Hence the generated substrate suitable larva is much too small, and the stag beetle can not develop in such rootstocks.

The devastating conclusion is, that 99% of the rootstocks in German forests are not suitable for the development of the stag beetle. Although there are plenty of rootstocks available, but the mayority of them is "protected" by their tannic acid content against fungal activity. These rootstocks are rather a reason for the untimely death of the females, desperately searching for suitable substrate for their eggs.

So the questions remains, why we do not simply change forest management from tree cutting in winter to cutting during summer? The cutting during wintertide is a concession to nature conservation as well. Spring and summer are breeding time for birds. In earlier times, many birds nests and young birds have been lost by tree cutting, for some species even at a critical rate. Switching to cutting in summer would surely help the stag beetle populations to recover quickly, however could cause new problems for various bird populations.

4.   Support programmes

Lucanus cervus, male in defensive stance So the question is, what can be done to preserve the species and to make sure, that to populations recover and the populated areas increase. Stag beetle loggeries can be regarded as an effective way to support the species. It is important, that these loggeries comply with the requirements of the stag beetle.

Lucanus cervus requires decaying oak wood colonized with the above mentioned fungi. The stag beetle loggery should be a group of large logs (more than 30 cm in diameter), sunk 30 to 50 cm into the ground, placed in a partially shaded area. Such a loggery should be protected against predators: Would hogs for example would quickly discover, what is hidden in the loggery and would purposefully rummage through the loggery to find and eat the larvae. Also badgers could dig for larvae and beetles, although the damage would be most probably smaller than those cause by wild hogs.

These damages can be avoided by using one of the models of stag beetle loggeries as described by TOCHTERMANN:

  1. Use available oak stumps with natural infestation with fungi. Cover them with a pile of oak wood chippings. The pile should be 40 cm in height. The diameter of the pile can be up to 4 m.

  2. Arrange decaying oak logs of at least 30 cm diameter in a 30 cm cavity in the ground to form a pyramid. The gaps between the logs should be filled with oak wood chippings and the pyramid covered with earth. The earth cover should be permeable to water.

  3. Sink decaying oak logs of at least 30 cm diameter into the ground (30-50 cm).

  4. Put pieces or slices of oak wood on the ground like a pavement. The total area should cover at least 10m2.

It should be noted, that building such a stag beetle loggery only makes sense, if a suitable forest is close by. Oak forests covering an area of at least 5 ha with trees with 150 to 250 years in age are especially suitable. Also stag beetles should be present in the area close by already. Although stag beetles can cover larger distances in flight especially on windy days, usually the loggery should be within a range of 2,000 m of an existing stag beetle population. It is also important to have oak trees with spots where sap is oozing from the bark within the same range around the loggery.

In a longterm perspective, a more sustainable and ecological forest management would be the best option. That would include to stay away from clear-cutting and to preserve a certain number of trees per hectare, which are relinquished to the cyle of growth and decay.

5.   Oddities

Lucanus cervus, male in defensive stance In extremely rare cases saggital semi-hermaphrodites can be found in stag beetle populations. In these specimen, one of the mandibles is build like in males whereas the other one is small, similar to those found in females. Normally the semi-hermaphrodites are male on on the left side and female on the right side, the opposite case is extremely rare. However, the specimen is often not a real hermaphrodite, as the genitalia can be clearly assigned to one gender.

Of course a large and eye-catching species like the stag beetle has a lot of vernacular names. Obviously there large mandibels are the origin of many vernacular names like stag beetle in English or Hirschkäfer in German. Another old vernacular name in German is 'Feuerschröter', indicating an association with fire. As the mandibles of the male are hollow and shine redish if exposed to a light source by night, our ancestors believed that the flying stag beetle had taken a piece of glowing wood from the fireplace to set a house on fire elsewhere.

Literature (in German only):

  1. BINOT, M. et al. (1998): Rote Liste gefährdeter Tiere Deutschlands
  2. FREUDE, H., HARDE, K. W., LOHSE, G. A. (1969): Die Käfer Mitteleuropas, Volume 8
  3. BRECHTEL, F. & H. KOSTENBADER (2002): Die Pracht- und Hirschkäfer Baden-Württembergs, p. 571-586
  4. KÖHLER, F. & B. KLAUSNITZER (1998): Verzeichnis der Käfer Deutschlands - Entomofauna Germanica, Dresden
  5. TOCHTERMANN, E. (1992): Das "Spessartmodell" heute, Neue biologische Fakten und Problematik der Hirschkäferförderung. - Allgemeine Forstzeitschrift, 47. Jahrgang, 6, p. 308-311